In a communications system, multiple narrowband signals (that is, a multi-subband signal) usually need to be received. Carrier frequencies (a carrier frequency may be represented as a frequency or an angular frequency) of all the narrowband signals are different from each other. In addition, bandwidths of all the narrowband signals may be the same or be different. A narrowband signal may also be referred to as a subband signal.
A technology of estimating, according to a multi-subband signal, an information symbol transmitted at each carrier frequency is generally referred to as a subband sampling technology. It is assumed that the multi-subband signal includes a maximum of k narrowband signals, and carrier frequencies of the k narrowband signals are different from each other. In this case, a set of hardware needs to be prepared for each carrier frequency of the k carrier frequencies. Each set of hardware includes two channels: one channel is used to perform sine wave frequency mixing, filtering, and sampling (analog-to-digital conversion) sequentially, and the other channel is used to perform cosine wave frequency mixing, filtering, and sampling sequentially. Frequencies of a sine wave and a cosine wave that are used on the two channels are the same and are equal to an angular frequency of a corresponding carrier frequency.
However, a sine wave signal or cosine wave signal that is used for frequency mixing is generated by a conventional analog device. This analog device generates a monophonic (that is, single-carrier) signal within any period of time, which requires as small a high-order harmonic wave as possible. Moreover, parameters of this analog device are preconfigured fixed values, and are difficult to adjust or modify. Therefore, when a receiver is being designed, a quantity of channels required in the receiver needs to be designed according to a maximum quantity of narrowband signals included in a multi-subband signal. For example, if the multi-subband signal includes a maximum of k narrowband signals, a receiver including 2k channels needs to be designed. Designing of this receiver lacks flexibility.